1. Field of the Invention
The present invention is in the field of hydraulics, and more specifically relates to hydraulic positioning devices of a locking type, which typically are used to control the tilt of seat backs in aircraft.
2. The Prior Art
The anti-surge valve of the present invention is an improvement for use in hydraulic locking devices such as those described in U.S. Pat. No. 3,380,561 issued to C. R. Porter, and U.S. Pat. No. 4,155,433 issued to C. R. Porter, both patents being assigned to P. L. Porter Company, the assignee of the present invention.
Hydraulic locking devices of this type include a moveable piston which separates two working chambers in a cylinder. The working chambers are normally filled to capacity with a hydraulic fluid, and movement of the piston is made possible by a selectively-enabled flow of fluid through a passage extending through the piston. A pressurized fluid reservoir supplies a small compensation flow into and out of the working chambers to compensate for changes in the total volume of fluid in the chambers due to leakage or thermal expansion. The compensation flow is enabled only when the piston is near one end of its stroke, and the compensation flow velocity is relatively slow because the reservoir is not highly pressurized.
Normally, one end of the hydraulic locking device is attached to a stationary member, and the other end of the hydraulic locking device is attached to a moveable structure which is to be selectively locked at a chosen position. In a typical application, the moveable structure is an arm connected to the tiltable back of a seat. When the seat back is pushed forward, the hydraulic locking device is extended in length.
Such devices include a control pushbutton connected to a control rod which opens a valve enabling flow through the piston, and no problems are encountered with this mode of operation. However, the embodiments of the hydraulic locking device with which the present invention is concerned further include means for operating in an override mode, wherein, when the seat back is pushed forward, pressure produced in one of the working chambers unseats the spring-loaded ball valve within the piston enabling flow of fluid through the piston even though the control pushbutton has not been actuated.
As the hydraulic locking device is being thus extended in the override mode, the pressure rises substantially and rapidly in the working chamber whose volume is being reduced, because of the viscosity of the fluid and the relatively small cross section of the flow passages through the piston. Near the end of the expansion stroke, a bleed orifice in the piston rod which communicates with the chamber whose volume is being reduced, arrives at the port in the cylinder wall that leads to the pressurized reservoir. The pressure in the reservoir is not as great as the transient pressure in the working chamber. When the bleed orifice becomes aligned with the port in the cylinder wall, the high transient pressure in the bleed orifice drives fluid into the reservoir, displacing the spring-loaded reservoir seal.
Because fluid is displaced into the reservoir instead of into the chamber whose volume is being increased, the total volume of fluid remaining in the working chambers is no longer equal to the total volume of the space of the working chambers. This results in a vacuum space being formed in the chamber whose volume is being increased. This vacuum space manifests itself as backlash or play of the seat back, i.e., inability of the hydraulic device to hold a definite position, with the result that the seat back can freely be moved within a small interval.
In addition to resulting in sloppy positioning of the seat back, the surge of high-pressure fluid into the reservoir displaces the spring-loaded reservoir seal, causing excessive wear of the seal.
Thus, the need was recognized for some means of preventing the high-pressure, high-velocity surge of fluid into the reservoir without interfering with the normal low-velocity compensation flow into and out of the reservoir.